1. Field
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device and a method for fabricating the same, to improve a contrast ratio of the LCD device by using a black matrix layer of resin.
2. Discussion of the Related Art
Demands for various display devices have increased with development of an information society. Accordingly, much effort have been expended to research and develop various flat display devices such as liquid crystal display (LCD), plasma display panel (PDP), electroluminescent display (ELD), and vacuum fluorescent display (VFD). Some species of flat display devices have already been applied to displays for various equipment.
Among the various flat display devices, liquid crystal display (LCD) devices have been most widely used due to advantageous characteristics of thin profile, lightness in weight, and low power consumption, whereby the LCD devices provide a substitute for a Cathode Ray Tube (CRT). In addition to mobile type LCD devices such as a display for a notebook computer, LCD devices have been developed for computer monitors and televisions to receive and display broadcasting signals.
Despite various technical developments in the LCD technology having applications in different fields, research in enhancing the picture quality of the LCD device has been, in some respects, lacking as compared to other features and advantages of the LCD device. In order to use LCD devices in various fields as a general display, the key to developing LCD devices depends on whether LCD devices can implement a high quality picture, such as high resolution and high luminance with a large-sized screen, while still maintaining lightness in weight, thin profile, and low power consumption.
A general LCD device includes an LCD panel for displaying a picture image, and a driving part for applying a driving signal to the LCD panel. The LCD panel includes first and second glass substrates being bonded to each other at a predetermined interval therebetween, and a liquid crystal layer injected between the first and second glass substrates.
The first glass substrate (TFT array substrate) includes a plurality of gate and data lines, a plurality of pixel electrodes, and a plurality of thin film transistors. The plurality of gate lines are formed on the first glass substrate at fixed intervals, and the plurality of data lines are formed in perpendicular to the plurality of gate lines at fixed intervals. Then, the plurality of pixel electrodes, arranged in a matrix-type configuration, is respectively formed in pixel regions defined by the plurality of gate and data lines crossing each other. The plurality of thin film transistors are switched according to signals of the gate lines for transmitting signals of the data lines to the respective pixel electrodes.
The second glass substrate (color filter substrate) includes a black matrix layer that excludes light from regions except the pixel regions of the first substrate, R(red)/G(green)/B(blue) color filter layer displaying various colors, and a common electrode to obtain the picture image. In case of an In-Plane Switching (IPS) mode LCD device, the common electrode is formed on the first glass substrate.
Next, a predetermined space is maintained between the first and second glass substrates by spacers, and the first and second substrates are bonded to each other by a seal pattern having a liquid crystal injection inlet. At this time, the liquid crystal layer is formed according to a liquid crystal injection method, in which the liquid crystal injection inlet is dipped into a vessel having liquid crystal while maintaining a vacuum state in the predetermined space between the first and second glass substrates. That is, the liquid crystal is injected between the first and second substrates by an osmotic action. Then, the liquid crystal injection inlet is sealed with a sealant.
Meanwhile, the LCD device is driven according to the optical anisotropy and polarizability of liquid crystal material. Liquid crystal molecules are aligned using directional characteristics because the liquid crystal molecules each has long and thin shapes. In this respect, an induced electric field is applied to the liquid crystal for controlling the alignment direction of the liquid crystal molecules. That is, if the alignment direction of the liquid crystal molecules is controlled by the induced electric field, the light is polarized and changed by the optical anisotropy of the liquid crystal, thereby displaying the picture image. In this state, the liquid crystal is classified into positive (+) type liquid crystal having positive dielectric anisotropy and negative (−) type liquid crystal having negative dielectric anisotropy according to electrical characteristics of the liquid crystal. In the positive (+) type liquid crystal, a longitudinal (major) axis of a positive (+) liquid crystal molecule is in parallel to the electric field applied to the liquid crystal. Meanwhile, in the negative (−) type liquid crystal, a longitudinal (major) axis of a negative (−) liquid crystal molecule is perpendicular to the electric field applied to the liquid crystal.
FIG. 1 is an exploded perspective view illustrating a general Twisted Nematic (TN) mode LCD device. As shown in FIG. 1, the TN mode LCD device includes a lower substrate 1 and an upper substrate 2 bonded to each other at a predetermined interval therebetween, and a liquid crystal layer 3 injected between the lower and upper substrates 1 and 2.
More specifically, the lower substrate 1 includes a plurality of gate lines 4, a plurality of data lines 5, a plurality of pixel electrodes 6, and a plurality of thin film transistors T. The plurality of gate lines 4 are formed on the lower substrate 1 in one direction at fixed intervals, and the plurality of data lines 5 are formed in perpendicular to the plurality of gate lines 4 at fixed intervals, thereby defining a plurality of pixel regions P. Then, the plurality of pixel electrodes 6 are respectively formed in the pixel regions P defined by the plurality of gate and data lines 4 and 5 crossing each other, and the plurality of thin film transistors T are respectively formed at crossing portions of the plurality of gate and data lines 4 and 5.
Next, the upper substrate 2 includes a black matrix layer 7 that excludes light from regions except the pixel regions P, R(red)/G(green)/B(blue) color filter layers 8 for displaying various colors, and a common electrode 9 for displaying a picture image.
At this time, the thin film transistor T includes a gate electrode, a gate insulating layer (not shown), an active layer, a source electrode, and a drain electrode. The gate electrode projects from the gate line 4, and the gate insulating layer (not shown) is formed on an entire surface of the lower substrate. Then, the active layer is formed on the gate insulating layer above the gate electrode. The source electrode projects from the data line 5, and the drain electrode is formed in opposite to the source electrode. Also, the aforementioned pixel electrode 6 is formed of transparent conductive metal having great transmittance, such as ITO (Indium-Tin-Oxide).
In the aforementioned LCD device, liquid crystal molecules of the liquid crystal layer 3 on the pixel electrode 6 are aligned with a signal applied from the thin film transistor T, and light transmittance is controlled according to alignment of the liquid crystal, thereby displaying the picture image. In this state, an LCD panel drives the liquid crystal molecules by an electric field perpendicular to the lower and upper substrates. This method obtains great transmittance and high aperture ratio. Also, it is possible to prevent liquid crystal cells from being damaged by static electricity since the common electrode 9 of the upper substrate 2 serves as the ground. However, in case of driving the liquid crystal molecules by the electric field perpendicular to the lower and upper substrates, it is difficult to obtain a wide viewing angle.
In order to overcome these problems, an In-Plane Switching (IPS) mode LCD device has been proposed recently. Hereinafter, a related art IPS mode LCD device will be described with reference to the accompanying drawings. FIG. 2 is a plane view of showing a unit pixel region of an IPS mode LCD device according to the related art. In the related art IPS mode LCD device, as shown in FIG. 2, a gate line 4 and a data line 5 crossing each other are formed on a transparent lower substrate, thereby defining a pixel region. Then, a thin film transistor is formed at a crossing portion of the gate and data lines 4 and 5. Also, a common line 4a is formed in parallel with the gate line 4 in the pixel region.
At this time, the thin film transistor includes a gate electrode of occupying one portion of the gate line 4, a gate insulating layer (not shown) formed on an entire surface of the lower substrate including the gate electrode, an active layer 10 formed on the gate insulating layer above the gate electrode, a source electrode 5a projecting from the data line 5, and a drain electrode 5b formed at a predetermined interval from the source electrode 5a. 
Also, a passivation layer (not shown) is formed on the entire surface of the lower substrate including the data line 5, wherein the passivation layer is formed of a silicon nitride layer. Furthermore, contact holes 11a and 11b are formed above the drain electrode 5b and the common line 4a. On the passivation layer of the pixel region, common electrodes 12 and pixel electrodes 13 are alternately formed in parallel at a predetermined interval.
At this time, the plurality of common electrodes 12 are formed within one pixel region in parallel with the data line, wherein each of the common electrodes 12 is connected with the common line 4a by the contact hole 11b. Also, each of the pixel electrodes 13 is connected with the drain electrode 5b of the thin film transistor by the contact hole 11a. The common electrode 12 and the pixel electrode 13 are formed of transparent conductive layers.
Although not shown, an upper substrate is formed opposite to the lower substrate, wherein the upper substrate includes a black matrix layer, color filter layers, and an overcoat layer. The black matrix layer prevents light leakage on remaining portions except the pixel regions, the color filter layers are provided for realizing colors in the respective pixel regions, and the overcoat layer is formed on an entire surface of the upper substrate including the color filter layers.
In the aforementioned LCD device, the upper substrate having the black matrix layer will be described in detail.
FIG. 3 is a plane view of the upper substrate of the LCD device according to the related art. FIG. 4 is a cross sectional view along I-I′ of FIG. 3.
As explained above, on the upper substrate 2, the black matrix layer 22 is formed on the remaining portions except the pixel regions 21. At this time, the black matrix layer 22 may be formed of a light-shielding metal material such as chrome Cr, or an acrylic resin formed by mixing carbon with a metal oxide material.
In the TN mode LCD device of FIG. 1, the black matrix layer 22 may be formed of the light-shielding metal material.
However, in case of the IPS mode LCD device of FIG. 2, if the black matrix layer 22 is formed of the light-shielding layer such as chrome Cr, residual images are generated. That is, in the IPS mode LCD device of FIG. 2, the liquid crystal is driven with the IPS mode electric field formed between the common electrode and the pixel electrode, in parallel with the two substrates. In this state, if the black matrix layer of the conductive metal is formed on the upper substrate, electrons are induced to the black matrix layer, thereby distorting the IPS mode electric field formed between the common electrode and the pixel electrode in parallel.
In order to prevent the distortion of the IPS mode electric field, the black matrix layer is formed of resin.
Generally, the LCD device having a luminance of 400 NIT requires the black matrix layer to have an optical density of 4.5 (or more). In the related art, if the black matrix layer 22 is formed of acrylic resin, the acrylic resin has an optical density OD of 3.0 to 4.0. Accordingly, light leakage is generated, thereby lowering the contrast ratio. Also, if the black matrix layer is formed of the acrylic resin, the black matrix layer is thickly formed to increase the optical density. In this case, step difference may be generated in the color filter layer.
As the luminance increases, this problem becomes more serious. That is, the LCD device for the television monitor has a full white luminance of 400 NIT to 600 NIT. Accordingly, for a high resolution LCD device, if the black matrix layer is formed of acrylic resin, the light leakage becomes serious.